1. Field of the Invention
The present invention relates to proximity signaling systems, and more particularly to collision avoidance systems adapted for railway use.
2. Description of the Prior Art
The use of an overhead line, a power bus, or the rails themselves of a track as a conduit for signals in a railway system has been known in the past. Such signals have served various purposes, including the purpose of automatic control over train density and collision avoidance. Often such control or proximity signals are present on the rails along with electrical power transmission which provides the motive power to the train and in such combination the power signal and the control signals are generally separated by frequency.
Most often, D.C. electrical power is used for traction and the control signals are in the form of alternating electrical signals superposed over the D.C. signal by either a single or a plurality of signal transmitters. In order to obtain good isolation between any noise associated with a D.C. signal source and the control signals, the transmitters connected to the track are typically operating at a relatively high frequency. Such transmitters, furthermore, were often connected to A.C. isolated segments of track in order to limit interference between transmitted signals thereof.
Thus the track generally was divided into A.C. isolated sections, each section connected to a control signal transmitter generating an alternating signal, each section being furthermore powered by a D.C. power source. As the train passed, or was within a particular section of track, a trainborne receiver would pick up the superposed control signals and in many applications would modify such control signal. A second train, therefore, appearing on the same segment of track would see an altered control signal and upon detecting the alterations therein would thus detect the presence of the other train.
The difficulty with the arrangement described above is that the track has to be segmented into discrete A.C. isolated sections and therefore instances can occur where two trains can be immediately adjacent across the A.C. isolation device and still not sense the relative presence of each other.
For that reason, a further improvement has been made in the recent past wherein the track sections are A.C. coupled and the impression of the A.C. signal thereon will therefore follow the normal loss with distance, or attenuation with distance, function as result of coupling between the conduits, and other losses. A train straddling the conduits would present additional attenuation and thus register its presence.
The difficulty with this arrangement, however, is that the loss or attenuation along the conduits combines with the other transmitted signals and the combination varies quite significantly with changes in atmospheric conditions and ground conditions between such conduits. Thus a direct reading of the signal amplitude does not provide a sufficiently accurate indication of the close presence of adjacent trains. For that reason, use of repeated single frequency transmissions has been abandoned in favor of a plural frequency transmission arrangement where transmission of one frequency is alternated with transmissions of other frequencies along the track. Thus within a particular segment of the track there will be at least two control signals, one possibly attenuated more than the other, which are, however, above a minimal amplitude necessary for sensing when not straddled by a train. Thus when both such signals are present, the sensors on the train identify a safe condition. For the purposes referred to herein, it is to be understood that the word "section" no longer refers to an A.C. or D.C. isolated segment of track, but is instead related to a section of track within which the control signal levels are above a predetermined amplitude.
Thus, systems have been devised in the past where at least two distinct frequency control signals are alternatively impressed onto the track, on a recurring basis, such that the conduits of the track always carry locally transmitted control signals above a predetermined signal level, unless shorted by the train. The repeated superposition of such signals onto the conduits, however, requires a plurality of independent external signal sources or transmitters which, although alternatively distinct in frequency, must be within a small frequency separation gap so that the normal frequency dependent attenuation thereof is substantially identical.
Thus, prior art level or amplitude responsive trainborne receivers are generally effective only if the two signal frequencies are close. Where the frequencies are close, the ability of isolating the two specific signals becomes critical. Thus, the trainborne sensors picking up such transmitted signals require narrow bandwidth filters, with which comes the problem of beats between the locally superposed signals and the more remote signal sources of the same frequency.